The present invention relates generally to controlling or driving alternating current (AC) motors. More particularly, the present invention relates to a method and apparatus for providing regenerative control of AC motors.
A wide variety of AC medium-voltage variable speed drives for induction motors are known which use a variation of current source topology with a phase-controlled silicon controlled rectifier (SCR) input stage and a 6-pulse or 12-pulse output. This topology may sometimes have the drawbacks of harmonic line currents, a variable power factor, and motor torque pulsations. These traits are especially problematic at higher power levels typical for medium voltage motor drives. Because of these and other disadvantages of the current source topology, pulse width modulated (PWM) circuits are preferred to provide motor control. Pulse width modulation is a form of modulation in which the value of each instantaneous sample of the modulating wave is caused to modulate the duration of a pulse. In PWM, the modulating wave may vary the time of occurrence of the leading edge, the trailing edge, or both edges of the pulse. The modulating frequency may be fixed or variable.
In a PWM circuit, a reference signal may be used to generate a train of pulses, the width of each pulse being related to the instantaneous value of the reference signal. The pulses may be generated by using a comparator to compare the reference signal with a carrier signal, which may be a saw tooth or triangular wave. When the reference signal exceeds the carrier signal, the output of the comparator is high; at other times, the output of the comparator is low. The comparator output does provide a train of pulses representing the reference signal. The pulses are then used to drive an electronic switching device for intermittently applying a voltage across the load.
U.S. Pat. No. 5,625,545, (hereinafter, the “'545 patent”) discloses a medium voltage PWM drive and method suitable for controlling medium voltage AC motors, in which a multi-phase power transformer having multiple secondary windings provides multi-phase power to each of a plurality of power cells. Each power cell has a single-phase output and is controlled by a modulation controller. Because the power cells are connected in series, the maximum output voltage for each cell may be less than the maximum line-to-line voltage.
Each power cell of the drive disclosed in the '545 patent may contain an AC-to-DC input rectifier, a smoothing filter, an output single phase DC-to-AC converter, and a control circuit. The input rectifier comprises a diode bridge that accepts three-phase AC input from a secondary winding circuit of the power transformer. The input rectifier transforms three-phase AC power into DC power that may have significant ripple. To ameliorate the effects of such ripple, a smoothing filter composed of electrolytic capacitors is connected to the DC side of the input rectifier. The smoothing filter also connects to the output converter. The output converter comprises a single-phase H-bridge of power transistors, such as, for example, insulated gate bipolar transistors (IGBTs). Each transistor of the output converter is operable by a local modulation control circuit. Signals for controlling the local modulation control circuit are provided by a master modulation controller. This allows the control of the power contribution of the power cell to the overall power supplied to the load.
As exemplified by the drive disclosed in the '545 patent, it is possible to produce a medium-voltage controller with low-voltage power cells by connecting multiple cells in series on each phase output line. Serial connection of cells makes multiple voltage states per phase possible; these multiple voltage states per phase may be used to obtain improved waveforms.
The pulse-width modulation technique of the '545 patent allows selective control of the duration and frequency of power cell pulse outputs. This method can use control signals, based upon interdigitated carrier signals, to selectively cause a switching event in a particular power cell. Typically, switching events are sequenced such that a switching event occurs in only one power cell at a time.
The techniques disclosed in the '545 patent are limited in that they provide a motor drive that cannot be used in a regenerative operation mode; i.e., a mode in which power flows from the motor through the drive to the transformer. A non-regenerative operation (or motoring) mode is operation in which power is supplied by the transformer through the drive to the motor. The master modulation controller in the '545 patent must carefully monitor the power flow in the system to avoid any situation in which power would flow from the motor into the drive. Failure to control the power flow would lead to an overvoltage situation developing within the power cell since the power cell has no means to dispose of power from the motor. The internal diode rectifier does not permit power to flow back to the AC mains connected to the drive. This effectively prevents the use of the drive in a regenerative mode.